Earth-Science Reviews 129 (2014) 120–135
Contents lists available at ScienceDirect
Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev
Neutron imaging of hydrogen-rich fluids in geomaterials and engineered
porous media: A review
E. Perfect a,⁎, C.-L. Cheng a,b, M. Kang a,c, H.Z. Bilheux c, J.M. Lamanna d, M.J. Gragg e, D.M. Wright b
a
Department of Earth and Planetary Sciences, University of Tennessee – Knoxville, Knoxville, TN 37996, United States
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
d
Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee – Knoxville, Knoxville, TN 37996, United States
e
Altamont Environmental, Inc., Asheville, NC 28801, United States
b
c
a r t i c l e
i n f o
Article history:
Received 15 July 2013
Accepted 25 November 2013
Available online 12 December 2013
Keywords:
Neutron imaging
Porous media
Radiography
Tomography
Water
Hydrocarbons
a b s t r a c t
Recent advances in visualization technologies are providing new discoveries as well as answering old questions with
respect to the phase structure and flow of hydrogen-rich fluids, such as water and oil, within porous media. Magnetic resonance and x-ray imaging are sometimes employed in this context, but are subject to significant limitations. In
contrast, neutrons are ideally suited for imaging hydrogen-rich fluids in abiotic non-hydrogenous porous media because they are strongly attenuated by hydrogen and can “see” through the solid matrix in a non-destructive fashion.
This review paper provides an overview of the general principles behind the use of neutrons to image hydrogen-rich
fluids in both 2-dimensions (radiography) and 3-dimensions (tomography). Engineering standards for the neutron
imaging method are examined. The main body of the paper consists of a comprehensive review of the diverse scientific literature on neutron imaging of static and dynamic experiments involving variably-saturated geomaterials
(rocks and soils) and engineered porous media (bricks and ceramics, concrete, fuel cells, heat pipes, and porous
glass). Finally some emerging areas that offer promising opportunities for future research are discussed.
© 2013 Elsevier B.V. All rights reserved.
Contents
1.
2.
Introduction . . . . . . . . . . . . . . .
Neutron imaging . . . . . . . . . . . . .
2.1.
Neutron transmission radiography . .
2.2.
Neutron computed tomography . . .
3.
Neutron imaging standards . . . . . . . .
4.
Imaging studies on geomaterials . . . . . .
4.1.
Rocks . . . . . . . . . . . . . . .
4.2.
Soils . . . . . . . . . . . . . . .
5.
Imaging studies on engineered porous media
5.1.
Bricks and ceramics . . . . . . . .
5.2.
Concrete . . . . . . . . . . . . .
5.3.
Fuel cells . . . . . . . . . . . . .
5.4.
Heat pipes . . . . . . . . . . . . .
5.5.
Porous glass . . . . . . . . . . . .
6.
Discussion and future directions . . . . . .
Acknowledgements . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . .
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132
1. Introduction
⁎ Corresponding author. Tel.: +1 865 974 6017.
E-mail address: [email protected] (E. Perfect).
0012-8252/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.earscirev.2013.11.012
As imaging technologies continue to improve our ability to visualize
the phase structure and flow of hydrogen-rich fluids at the pore scale,
the resulting high resolution data sets provide opportunities for
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
evaluating existing models, and developing new theoretical frameworks. While magnetic resonance imaging (MRI) and x-ray imaging
continue to be employed in this context, both techniques are subject
to significant limitations. For example, x-ray imaging relies on the use
of tracers to differentiate between air and water in variably-saturated
porous media (Basavaraj and Gupta, 2004), while MRI is limited by
the range of pore sizes that can be visualized (Chen et al., 2003) and
the presence of iron in the solid matrix (Hall et al., 1997). In contrast,
neutrons are ideally suited for this application because of their strong
attenuation by hydrogen in water and oil, and their relative insensitivity
to both the gas phase in pores and solid constituents, such as silica and
iron.
Brenizer (2013) has reviewed the history of neutron imaging from its
conception to the present day. The neutron itself was discovered by James
Chadwick in 1932. Only 3 yr later Hartmut Kallmann and Ernst Kuhn in
Berlin, Germany began to make radiographic images of objects using neutrons. However, little progress was made on neutron imaging until the
1950's when technical improvements in the film employed opened up
the field to practical applications. Neutron imaging is based on measuring
the transmitted intensity of neutrons through an object, either in two dimensions (radiography) or three dimensions (tomography).
In the geosciences neutrons were initially used to measure the
water content of soil. Research based on the thermalization of
“fast” neutrons released from a source probe inserted into an access
tube commenced in the 1950's following the seminal paper by
Gardner and Kirkham (1952). It was not until the 1970's, however,
that neutron imaging was first applied to natural and engineered porous media (Reijonen and Pihlajavaara, 1972; Subraman and
Burkhart, 1972; Wilson et al., 1975; Lewis and Krinitzsky, 1976). Although there have been previous reviews of this topic with respect to
applications in earth science, material science, and engineering
(Lehmann et al., 2004; Wilding et al., 2005; Winkler, 2006; Banhart
et al., 2010; Hess et al., 2011; Kardjilov et al., 2011), none of these focused specifically on neutron imaging of hydrogen-rich fluids in
variably-saturated porous media.
This review paper provides an overview of the general principles behind the use of neutron radiography and tomography. The standards for
neutron imaging are also examined. The main body of the paper consists
of a comprehensive review of the diverse scientific literature on neutron
imaging of static and dynamic experiments involving hydrogen-rich
fluids in variably-saturated abiotic porous media. We consider the following natural and engineered materials: bricks, ceramics, concrete,
fuel cells, heat pipes, porous glass, rocks, and soils. The focus is on
nano-, micro- and meso-scale porous systems in which capillary forces
dominate over gravity. Research on macro-porous materials such as aircraft wings with an internal honeycomb structure (Hungler et al., 2009),
or biological materials such as plant roots and wood xylem tissue (e.g.,
Nakanishi and Matsubayashi, 1997), is beyond the scope of this review
and will not be covered. Finally, some new developments in neutron
imaging that offer exciting opportunities for future research will be
discussed.
2. Neutron imaging
Neutron imaging beamlines have traditionally been installed at
reactor-based facilities, although a few are associated with spallation
sources. Table 1 lists the most well-known existing neutron imaging
facilities, along with their beamline parameters. Many of these have
been in operation for decades. Over time, two main factors have ensured
a rapid increase in neutron imaging capabilities and applications:
(1) higher neutron fluxes at some facilities, and (2) advances in digital
imaging. As a result, thermal neutron fluxes can be as high as
108 n cm−2 s−1, while the use of charge-coupled device (CCD) cameras
allows for 2-dimensional (2D) real-time radiographs with spatial resolutions of up to ~15 μm (Table 1).
121
2.1. Neutron transmission radiography
Neutron transmission radiography (NTR) is a non-destructive, noninvasive 2-dimensional (2D) imaging technique based on the attenuation (absorption and scattering) of a neutron beam as it passes through
a sample, as illustrated in Fig. 1. The resulting “flat” image is a map of the
neutron attenuation within the sample under investigation. Neutrons
interact with the nucleus of the atom rather than with its electron
cloud. The interaction forces between neutrons and nuclei are not correlated with the atomic number of the element, but instead depend upon
the particular isotope of the element (Anderson et al., 2009; Strobl et al.,
2009). For example, neutrons are highly sensitive to light isotopes such
as 1H, 6Li, 10B, and rather insensitive to heavier isotopes such as 82Pb.
For a monochromatic (single wavelength) beam traversing a homogeneous sample, the measured intensity, I, is given by the Lambert–Beer
law (Anderson et al., 2009):
−μτ
I ¼ I0 e
ð1Þ
where I0 is the incident beam intensity, μ is the attenuation coefficient in cm− 1 and τ is the sample thickness. In the case of a polychromatic neutron beam going through a heterogeneous sample
comprised of n elements, Eq. (1) becomes:
Z
I ðλÞ ¼
λmax
λmin
−Σn ðτ μ ðλÞdλ
I0 ðλÞe½ i¼1 i i
ð2Þ
where λ is the neutron wavelength, τi is the thickness of element i,
A
and μ i ðλÞ ¼ σ i ðλÞmρN
is the linear attenuation coefficient of element
M
i, where σi (λ) is the microscopic cross section of element i, m is
the number of moles of a molecule, ρ is the density, M is the molecular weight, and NA is the Avogadro constant.
Both absorption and scattering influence the level of contrast in a 2D
image. Imaging a thick sample with a polychromatic neutron beam can
result in artifacts due to beam hardening (Hassanein, 2006). As the
beam passes through the sample, its mean energy increases (i.e., it becomes “harder”) because the lower-energy neutrons are preferentially
absorbed, leaving behind only the higher energy neutrons.
As indicated by Eq. (2), the contrast mechanism strongly depends
upon the radiation source, i.e. the range of neutron wavelengths available
at the beamline. Using the different neutron wavelengths at pulsed spallation sources it is possible to obtain multiple radiographs of the same
sample, each with very different contrasts (a kind of “multispectral” imaging known as time-of-flight imaging).
The following worked example illustrates the impact of two different
wavelengths on neutron transmission. First order approximations of the
attenuation coefficients for water (H2O) in thermal (1.54 Å) and cold
(9 Å) monochromatic neutron beams can be calculated based on Eq. (3):
μ H2 O ¼ σ ðHÞ 2 ρðH2 OÞ N A =MðH2 OÞ þ σ ðOÞ ρðH2 OÞ NA =M ðH2 OÞ
ð3Þ
where σ(H) = 82 barn and σ(O) = 4 barn at 1.54 Å (National Institute of
Standards and Technology, 2013), σ(H) = 110 barn and σ(O) = 6 barn at
9 Å (Brookhaven National Laboratory, 2013), ρðH2 OÞ ¼ 1 g cm−3,
NA = 6.022 × 10−23 mol−1, and M ðH2 OÞ ¼ 18.02 g mol−1. The resulting
values for μ H2 O are 5.62 cm−1 and 7.55 cm−1 for thermal and cold neutrons, respectively. Using these values in Eq. (1) gives the neutron transmission (I/I0) as a function of water thickness. Fig. 2 shows the
transmission curves for water in thermal and cold monochromatic neutron beams assuming no scattering effect. For any given water thickness,
attenuation of the cold neutron beam is greater than with the thermal
neutron beam.
Secondary scattered neutrons, as well as background from the environment, can also cause artifacts in the levels of contrast in radiographic
images (Hassanein, 2006). The errors produced by scattering and background are often much larger than those due to beam hardening
122
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
Table 1
Prominent neutron imaging facilities and their beam line characteristics.†
Country
Institution
Facility/instrument
Reactor Flux “on sample” L/D
power (n cm−2 s−1)
(MW)
Australia‡
Austria
ANSTO§
Institute of Atomic and Subatomic
Physics/Vienna University of Technology
IPEN
Royal Military College of Canada
OPAL (DINGO)
TRIGA II
20
0.25
5 × 107
1 × 105
IEA-R1
Slowpoke-2
5
0.02
CIAE
PKU
HZB
CARR
PKUNIFTY
CONRAD (BENSC BER-II)
FRM-II
FRM-II
Brazil
Canada
China‡
Germany
Hungary
Japan
KFKI-AEKI
JRR-3M
Kyoto University
Korea
HANARO
South Africa NECSA
Switzerland
USA
§
250–1000
45–125
20–30
50–100
20 × 20
90 cm dia.
1 × 106
3 × 104
55–110
100
NA
140–300
60
4.5
10
8 × 107
5 × 105
6 × 106
160–3000
25–200
170–500
150
300
50
ANTARES
25
9 × 107
200–8000
50–200
NECTAR
25
3 × 107
150–300
600
WRS-M
TNRF/TNRF-2
KUR
NR-port
SANRAD
(SAFARI-1)
10
20
5
30
20
105
108
106
107
107
100
125–450
100
190–270
150–500
NA
20–60
200
NA
50–100
250–850
50–500
20
15–50
25 cm dia.
20 × 20
60 × 60
10 × 10
20 × 20
20 × 20
30 cm dia.
40 × 40
20 cm dia.
30 × 30
20 cm dia.
25 cm dia.
25 × 30
16 cm dia.
25 × 30
10 × 10; 13 × 13;
25 × 25
36 cm dia.
40 cm dia.
30 × 30
15 cm dia.
25 × 25
10 × 10
20 × 25
25 cm dia.
4×4
6×6
35 × 43
23 cm dia.
23 cm dia.
6
2.6
1.2
1
1
×
×
×
×
×
SINQ
(NEUTRA)
SINQ
(ICON)
National Institute of Science and Technology NCNR BT-2
1.4
4 × 106
1.4
7
20
6 × 10
100–
10,000
100–6000
Oak Ridge National Laboratory
85
2 × 107
Cornell University
‡
Field of view (rectangle,
cm x cm; circle, cm dia.)
PSI
McClellan Air Force Base Nuclear radiation
Centre
Penn State University
†
Spatial
resolution
(μm)
HFIR CG-1D
MNRC
2
(TRIGA)
2
Radiation Science and
Engineering Center
(Penn State Breazeale Reactor)
TRIGA Mark II
0.5
1 × 10
7
400–800
50–100
7
50–400
25–50
7
3 × 10
50–100
115–155
30–60 (2D)
130–150 (3D)
6 × 106
70–130
125–450
2 × 10
410 cm2
Sources: Anderson et al. (2009), de Beer and Radebe (2012), IAEA (2009, 2013); ISNR (2010), Lehmann et al. (2011b), ORNL (2012); Deinert et al. (2005b); ITMNR-7 (2013).
Facility under development.
Abbreviations:
ANSTO: Australian Nuclear Science and Technology Organization
IPEN: Institute. De Pesquisas Energetiscas e Nucleares
IAEA: International Atomic Energy Agency
HFIR: High-Flux Isotope reactor
HZB: Hellmholtz-Zentrum Berlin
CONRAD: Cold Neutron RADiography
ANTARES: Advanced Neutron Tomography And Radiography Experimental System
TNRF: Thermal Neutron Research Facility
HANARO: High-Flux Advanced Neutron Application Reactor
FRM-II: Forschungs-Neutronenquelle Heinz Maier-Leibnitz research reactor Munich II
CIAE: China Institute of Atomic energy
PKUNIFTY: Peking University Neutron Imaging. Facility
(Hassanein, 2006). Samples that are very thick or have a strong scattering
cross section (i.e., H2O) will have a greater probability that the scattered
neutrons will hit the detector. The angular distribution of the scattered
neutrons can result in significant deviations from the Lambert–Beer
Law, Eq. (1), depending upon the sample to detector distance (Radebe
et al., 2011; Kang et al., 2013a).
The effects of secondary scattering are usually most pronounced
in samples with large water thicknesses located close to the detector. Hassanein et al. (2005) indicated that errors due to secondary
scattering might be more than 45% for water with a thickness of
4 mm. In contrast, Hussey et al. (2010) employed random uncertainty analyses based on neutron counting statistics and concluded
that scattering effects at the neutron imaging facility of the National
Institute of Standards and Technology (NIST), Gaithersburg, MD are
limited.
OPAL: Open Pool Australian Lightwater reactor
TRIGA: Training, Research, Isotopes, General Atomics reactor
SLOWPOKE: Safe Low-Power Kritical Experiment
NCNR: NIST Center for Neutron Research
SANRAD: South African Neutron. Radiography
KFKI-AEKI: KFKI-Atomic Energy Research Institute
JRR-3M: Japan Research Reactor No.3 Modified
KUR: Kyoto University Research Reactor
PSI: Paul Scherrer Institute
NFNBR: National Facility for Neutron Beam Research
CARR: China Advanced Research Reactor
NECSA: South African Nuclear Energy Corporation
NA: Data not available
Different approaches have been proposed to remove scattering
effects. One is to restrict measurements to very thin sections of
water (Hussey et al., 2010) or very low water contents (Kim et al.,
2012). Another approach is to correct for the scattering using
Monte Carlo modeling of point scattering functions (Pleinert et al.,
1998; Hassanein et al., 2005, 2006b). The Quantitative Neutron Imaging (QNI) software program was developed for this purpose and
has been shown to correct for nonlinearities in water calibration
data (Radebe et al., 2011). However, modeling does not provide a
universal fix and alternative solutions such as experimental determination of the scattering component need to be investigated
(Hassanein et al., 2006b). Until a better solution is developed by
the neutron imaging community, investigators must rely on empirical water thickness calibrations to take into account the beam hardening, secondary scattering, and background effects associated with
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
a particular beam line/sample configuration (Tumlinson et al., 2008;
Kang et al., 2013a).
To minimize scattering artifacts, samples are often positioned a few
to several centimeters away from the detector. However, this protocol
produces an unsharpness effect which can be detrimental to the best
achievable spatial resolution. For static measurements (i.e., with no
motion unsharpness), two types of unsharpness are present: (i) the detector system unsharpness (due to optical components, such as lenses
and mirrors at the detector), and (ii) the geometric unsharpness (due
to the beamline optics, i.e. cone beam geometry instead of ideal parallel
beam geometry).
The geometric unsharpness is a measure of the loss of spatial resolution. The highest achievable spatial resolution is obtained when a thin
sample is placed against the detector (i.e., where the image is formed).
The spatial resolution decreases as the sample is moved away from the
detector, either because of its thickness or to minimize scattering effects. This loss in spatial resolution can be quantified by the geometric
unsharpness, Ug, given by (ASTM, 2012: E94-04, E2698-10; ISO, 2013:
19232-5):
L
U g ¼ l=
D
½4
where l is the sample to detector distance, D is the beam optics defining
aperture diameter, and L is the distance between the aperture and the
detector. The L/D ratio is often referred to as the figure of merit for a
neutron imaging beamline and corresponds to the apparent focal spot
size. In practice, the actual L/D ratio is different from the physical ratio
because of wall scattering in the collimator or guide, which can affect
the anticipated resolution (Brenizer, 1992). A large L/D ratio improves
spatial resolution at the cost of flux, which is not always practical. The
prospect of using coded apertures (Skinner, 1984; Caroli et al., 1987) instead of a single aperture may allow for large L/D ratios without loss of
flux, and ultimately promises high spatial resolution measurements
(Xiao et al., 2009; Zou et al., 2011).
123
oily shale, or the 3D distribution and interconnectivity of hydrogenrich fluids such as water and oil within the pore spaces of nonhydrogenous materials.
Neutron tomographic images are generally reconstructed assuming
parallel beam projection (Vontobel et al., 2006). After normalization of
the 2D radiographs, the Radon transform (Radon, 1917) is applied to
the data, producing sinograms. The results are then “back-projected” to
the sample and cross section (or reconstructed) slices of the sample are
obtained. Fig. 3 shows the Radon transform, Pθ(t), of an object f(x,y) for
parallel beam geometry and the corresponding Fourier transform,
Sθ(ω), of Pθ(t) based on the Fourier slice theorem. The projections,
Pθ(t), of an object f(x,y) rotated by an angle of θ along the beam path
are given by:
Z Z
P θ ðt Þ ¼
∞
−∞
f ðx; yÞδðxcosθ þ ysinθ−t Þdxdy
ð5Þ
where t = xcosθ + ysinθ. According to the Fourier slice theorem, the
1-dimensional (1D) Fourier transform of a parallel projection is equal
to a slice of the 2D Fourier transform of the object, i.e.
Z
Sθ ðωÞ ¼
∞
−i2πωt
−∞
P θ ðt Þe
Z Z
dt ¼
∞
−∞
−i2πωðxcosθþysinθÞ
f ðx; yÞe
dxdy ¼ F ðu; vÞ ð6Þ
where (u,v) = (ωcosθ, ωsinθ), ω = frequency, and F(u,v) is the
Fourier transform of f(x,y). The object at the point f(x0,y0) can then
be reconstructed by simply performing a 2-dimensional inverse
Fourier transform of the projection data, i.e.
Z Z
f ðx0 ; y0 Þ ¼
∞
−∞
−i2πðx0 uþy0 uÞ
F ðu; vÞe
dudv
ð7Þ
Computed tomography (CT) is a 3-dimensional (3D) reconstruction
of an object based on a series of 2D projection images (radiographs) acquired at different angles; 0 to 180° assuming a perfectly parallel beam,
or 0 to 360° for the best CT performance. This process is facilitated by
mounting the object on a stage that can be rotated during imaging. CT
reconstructions provide information about the 3D geometry/topology
of a sample, and the principles behind CT imaging are similar for both
x-ray and neutron sources. Neutron CT (NCT) can reveal either the
internal structure and texture of hydrogen-rich solid materials such as
The sequence of calculations described above is implemented in the
filtered back projection (FBP) algorithm, which is the most commonly
used CT reconstruction method (Kak and Slaney, 2001). An alternative
technique, known as the iterative reconstruction algorithm, has been
developed to help reduce noise and improve image quality (Vontobel
et al., 2006). Strobl et al. (2009) suggested using scans over the range
of 0 to 360° for large samples and a cone-beam reconstruction algorithm
to improve image quality due to deviations from perfect parallel neutron beam geometry.
The transverse images resulting from the reconstruction process are
combined to produce a 3D volume rendering of the sample. The 3D volume rendering converts pixels to voxels using linear interpolation of
two consecutive cross-section slices. Qualitative and quantitative data
analyses are often undertaken on transverse, sagittal or coronal slices
obtained from the volume rendering.
Fig. 1. Layout of a typical neutron imaging beamline (Reprinted with permission from
Nanda et al., 2012. Copyright 2012 American Chemical Society).
Fig. 2. Transmission (I/I0) as a function of water thickness for two different neutron energy
levels.
2.2. Neutron computed tomography
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E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
As a polychromatic neutron beam passes through an object, its attenuation depends on the elemental composition of the material and
the energy spectrum of the neutrons. Low energy neutrons will not
transmit as much as higher energy ones, which causes deviations
from the Lambert–Beer Law referred to as beam hardening (see
Section 2.1). During data analysis, careful attention must be paid to
beam hardening and scattering effects, the image unsharpness, and
the evaluation of experimental errors (Hassanein et al., 2005).
3. Neutron imaging standards
Two groups are involved in producing standards relevant to neutron
imaging, namely the International Organization for Standardization
(ISO) and the International Committee for Non-Destructive Testing
(E07) of the American Society for Testing and Materials (ASTM).
Brenizer (1992) reviewed the standards produced by these two groups
up to and including the early 1990's. At that time ISO was considering
standards for neutron radiography, but none had been produced.
Since then ISO has published standards on the principles and rules for
thermal neutron radiographic testing, 11537:1998 (ISO, 2013) and for
determining the beam L/D ratio value in thermal neutron radiography,
12721:2000 (ISO, 2013). ISO also has a standard for determining
image unsharpness in radiographs that is applicable to neutron radiography: 19232-5:2013 (ISO, 2013).
The E07 Committee of ASTM currently has seven standards specific
for neutron radiography: E545-05, E748-02, E803-91, E1496-05,
E2003-10, E2023-10, and E2861-11 (ASTM, 2012). These standards provide introductory material, basic guidance, common practices, and test
methods related to film-based NTR. Other ASTM standard practices
and guides, such as E94-04 and E2698-10 (ASTM, 2012), provide useful
information on image quality, precision, and potential bias (e.g., geometric unsharpness, distortions, and backscattering due to variations
in object-to-film/source-to-object distances) for both film-based and
digital radiography.
While both ISO and ASTM have developed generic CT standards (ISO,
2013: 15708-1/2:2002 and ASTM, 2012: E1441-11, E1570-11, E167212, E1695-95), there are currently no standards that deal specifically
with NCT. However, a recent initiative by the International Atomic Energy Agency (IAEA) in collaboration with the Paul Scherrer Institut,
Switzerland, Necsa, South Africa, and the Korea Atomic Energy Research
Institute, South Korea involves the evaluation of a set of test objects to
provide a standardized method to quantify the tomographic capabilities
of neutron imaging beamlines worldwide (Kaestner et al., 2013).
4. Imaging studies on geomaterials
4.1. Rocks
In rocks some water may be incorporated into the mineral structure of the solid phase. Neutron imaging has been applied to visualize the distribution of this “structural” water. For example, Winkler
et al. (2002) utilized NCT to analyze hydrous mineral growth in pegmatite granite, basanite, and garnet-mica schist samples. Structural
water does not contribute significantly to variable saturation. However, it must be accounted for by normalizing images acquired during wetting or drying with respect to images of the initially-dry
solid matrix.
Most of the mobile water in rocks occurs in the void spaces between
assemblages of mineral grains. The volume percentage of rock that is
void space between grains is the primary porosity. In addition to the primary porosity, fractures in rocks create secondary porosity. The total
porosity, ϕ, is the sum of the primary and secondary porosities, and typically ranges between 0.02 and 0.30 (Table 2) depending upon factors
such as rock type, diagenesis, weathering, and fracturing. The specific
surface areas of consolidated rocks are generally relatively small
(Table 2). In contrast, some unconsolidated rocks can have specific surface areas in the same range as those for soils.
The intrinsic permeability, k, of rocks can range over several orders of
magnitude (Table 2), depending on total porosity, pore-size distribution,
pore shape, and pore connectivity. These factors are highly dependent
on rock type, with unfractured igneous rocks generally having the lowest k values, and karst limestone the highest.
Over the past 20 yr or so, several studies have assessed the utility of
neutron imaging for visualizing the distribution of pore water in rocks
under equilibrium (or static) conditions. Kupperman et al. (1990) and
Rhodes et al. (1992) tested the feasibly of using neutrons to image
water in tuff samples for permanent disposal of radioactive waste. They
employed a dual energy technique with relatively coarse spatial resolution. Pleinert and Degueldre (1995) used NTR to determine the total porosity of crystalline rock (granodiorite and mylonite) samples saturated
with water. The same group also used NTR (as a complementary technique to positron emission tomography) for imaging a cylindrical core
of granodiorite rock into which holes of varying diameters had been
bored (Degueldre et al., 1996). These simulated pores were filled with
hardened hydrogen-rich epoxy resin, allowing visualization of flow
paths and determination of the total porosity. Solymar et al. (2003a)
used NCT to relate variations in water content after air flushing an
Fig. 3. (a) Radon transformation, Pθ(t), of an object f(x,y) at an angle of θ with t = xcosθ + ysinθ assuming parallel beam geometry, and (b) Fourier transformation, Sθ(ω), of the
projection Pθ(t), where (u,v) = (ωcosθ, ωsinθ) and ω = frequency.
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
125
Table 2
Typical value ranges for selected physical properties of various abiotic porous media.
Porous medium
Solid phase description
Specific surface area
(m2/g)
Porosity
(m3/m3)
Intrinsic permeability (m2)
Sources
Bricks and
ceramicsa
Formed materials (e.g., building bricks, pottery)
prepared by heating (and subsequently cooling)
selected soil fractions (e.g., clay)/crushed rocks
in the presence of water
Construction material made by mixing together
crushed rocks/selected soil fractions (e.g., sand),
cement (a mixture of oxides of calcium, silicon
and aluminum), and water
Typically carbon fiber paper or woven cloth that
has been wet-proofed with PTFE. Can be coated
on one side with a microporous layer formed
from a mixture of carbon black, polymer binder,
and hydrophobic agents.
Sintered powder, grooved tube or screen mesh
structures fabricated using metal (e.g., steel,
aluminum, nickel, copper) foams and felts, carbon
fibers, polymers, and ceramics
Glass containing silica (~96%), boron and sodium
oxides (~3%) produced by phase separation and
liquid extraction with pores ranging between
0.4 nm and 1 μm
Mixture of consolidated or unconsolidated primary
minerals, commonly including calcite, feldspar, mica,
quartz, and/or silica
Unconsolidated mixture of primary (e.g., feldspar,
mica, quartz) and secondary (e.g., kaolinite,
montmorillonite) minerals, plus organic matter
0.6–12.3
0.19–0.43
5 × 10−18–3.3 × 10−14
Xu et al. (1997), Dondi et al.
(2003), Kyritsis et al. (2009)
4.4–200
0.12–0.19
9.4 × 10−18–5.7 × 10−16
Lydon (1995), Tsivilis et al.
(2003), Sani et al. (2005),
Odler (2003)
1–120
0.44–0.90
9.6 × 10−14–1.6 × 10−9
Ihonen et al. (2004), Williams
et al. (2004), Gostick et al.
(2006), Holley and Faghri
(2006), Song et al. (2006)
0.06–75.0
0.15–0.97
7.3 × 10−13–1.7 × 10−9
70–200
0.28–0.35
0.2 × 10−19–1.8 × 10−19
Canti et al. (1998), Holley and
Faghri (2006), Huang et al.
(2009), Shkolnikov et al.
(2010)
Elmer (1992), Bentz et al.
(1998), Gelb and Gubbins
(1998), Gruener et al. (2012)
0.22–4.9
0.02–0.30
2.2 × 10−19–3.9 × 10−9
0.8–143.2
0.17–0.45
8.4 × 10−12–2.8 × 10−10
Concretea
Fuel cell gas
diffusion
layera
Heat pipe wicka
Porous glassa
Rocksb
Soilsb
a
b
Churcher et al. (1991),
Hammecker and Jeannette
(1994), Labrie and Conlon (2008)
Currie (1966), Fish and Koppi
(1994), Pennell et al. (1995)
Man-made (engineered) material.
Naturally-occurring geomaterial.
initially water-saturated sandstone core at a pressure of ~50 kPa to the
distribution of coarse and fine laminae.
de Beer et al. (2004a) and de Beer and Middleton (2006) determined
the porosity of water-saturated sandstones using NTR. These authors
obtained a ~1:1 relationship between porosity measured by NTR versus
results from a conventional measurement method for values of ϕ b 20%
(Fig. 4). Above ϕ = 20% the data deviated from a 1:1 relation because of
the detrimental scattering effects of neutrons onto the detector at high
water contents resulting in the underestimation of porosity (Fig. 4). de
Beer et al. (2004a) also analyzed iron ores, which often contain
hydrogen-bearing minerals such as limonite and goethite, and compiled
a list of attenuation coefficients of elements and compounds relevant for
imaging water in different rock types.
Many imaging experiments involving the movement of water and
other fluids in rocks under dynamic conditions have been done with either the preservation of natural building stone in mind, or for purposes
of reservoir characterization in petroleum engineering. Jasti et al. (1987)
were the first to acquire images of water moving in rock pores using dynamic NTR. These authors flooded an initially mineral oil-saturated
Berea sandstone core with water in order to observe the migration of
the immiscible front, while Jasti and Fogler (1992) recorded fluid distribution changes due to a miscible tracer pulse in flooding experiments
performed on Berea sandstone cores.
Middleton and Pàzsit (1998) and Sváb et al. (2000) used dynamic
NTR to investigate oil displacing heavy water in samples of Visingsö
sandstone from Sweden. Members of this same research group also
presented three different petrophysical applications of neutron imaging
involving water movement in rocks (Middleton et al., 2001). The
experiments imaged were: (1) vertical water infiltration in to an
initially-dry porous rock, (2) oil flooding of a sandstone rock initiallysaturated with heavy water, and (3) water flooding of an initially-dry
clay-rich rock. Solymar et al. (2003b) performed oil–water immiscible
displacement experiments at the same facility. The samples were Greensands (glauconite sandstones) from the North Sea, which are of interest
as oil reservoirs. Flow of oil displacing water through the pore space of
the samples was recorded by a low-light TV camera connected to a
super VHS recorder. Images were collected into stacks of 10 over
~0.5 s, a time resolution better than the fluid front advancement, and
stored as 8-bit grayscale images. Piston-like displacements were observed in samples with narrow pore-size distributions, while flow
channeling occurred in more heterogeneous samples.
Middleton et al. (2005) investigated spontaneous imbibition of
water into air-filled Mardie Greensand and Barrow group sandstone
samples using dynamic NTR. The observed data were fitted to a simple
diffusion equation with a constant diffusion parameter. In addition to
the static porosity measurements discussed previously, de Beer and
Middleton (2006) also imaged water displacing oil in Fontainbleu sandstone using a Hassler Cell setup. “Heavy water” (D2O) was used instead
of H2O, to enhance the detectable neutron intensity contrast between
the two fluid phases. The relative concentrations of the two fluids
could be quantified yielding the position of the D2O front as a function
of time.
In another dynamic study, Hassanein et al. (2006a) imaged the capillary imbibition of deionized water and a 20% solution of NaCl into
initially-dry rock samples (Mansfield sandstone, Salem limestone, and
Hindustan whetstone) of various sample sizes, up to 40 cm. The movements of the fluids were imaged over several hours, with exposure
times of 15–25 s per frame and a pixel resolution of 272 μm. Scattering
effects caused the water contents to be underestimated. Following correction by Monte Carlo modeling of the point scattered function, the
wetting front position was plotted as a function of the square root of
time, the slope of which yields the sorptivity, S. Their results show variations in S as a function of rock type, sample size, solution type, and
mode of imbibition (top down versus bottom up). Cnudde et al.
(2008) further explored the usefulness of high-speed NTR for quantifying water uptake in porous rocks by capillarity.
Kang et al. (2013b) estimated the sorptivity and unsaturated diffusivity of Berea sandstone from neutron radiographs acquired continuously
during spontaneous imbibition. Their estimates appear to be the first reported values of these hydraulic parameters for this important rock type,
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E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
which is used widely as a standard for petrophysical investigations in the
geology and petroleum engineering fields.
Polsky et al. (2013) investigated the applicability of neutron imaging
for enhanced geothermal system applications. They visualized an air–
water interface moving within a fracture in granite. Radiographic images were acquired with a resolution of approximately 300 μm and
10 ms exposure time. Relatively high fidelity flow structure details
could be resolved, including the curvature of the interface. Hall (2013)
employed time lapse NTR to quantify differential water imbibition
into an air dry sandstone core. Local fluid flow velocities, extracted
from image analysis of the neutron radiographs, indicated that water
flow was faster within a compactant shear band. This behavior was
attributed to higher capillary forces associated with damage such as increased microcrack density.
In terms of NCT, several experiments involving the dynamics of fluid
flows in rocks have been reported. Masschaele et al. (2004) and Dierick
et al. (2005) presented tomographs for water and other fluids moving
into limestone and sandstone rock samples, some of which had been
treated with a water repellent. Each tomograph was constructed from
100 projections, 10 frames/s over 10 s, at a resolution of 8 pixels or
1.5 mm. The dynamic processes imaged were relatively slow compared
to the high speed imaging capability. The capillary imbibition of water
repellents, consolidants (fluids, which upon hardening, reestablish cohesion between particles of deteriorated building stone), and gasoline,
as well as the effects of water migration on the porous rocks, were
shown as examples.
Hameed et al. (2009) and Zawisky et al. (2010) conducted neutron
imaging experiments (both NTR and NCT) to compare the uptake of
two different consolidants by porous building stone used in historic
buildings. Samples were soaked in a bath of consolidant. Uniform penetration of the consolidant into the samples was expected. However,
neutron imaging revealed that the consolidant was heterogeneously
distributed, with pronounced surface effects (Zawisky et al., 2010).
Wilding et al. (2005) reported several geological applications of neutron tomography. In terms of fluid flow, these authors presented NCT
reconstructed images of CO2 reaction fronts in carbonate rocks. They
also investigated water flow into a fractured volcanic ash deposit, the
Bandelier Tuff, from Los Alamos, NM. The hydrologic properties of this
rock are of interest in addressing the environmental impacts of possible
radioactive nuclide contamination at Los Alamos National Laboratory.
Ten mL of water was added to the top of the core sample and allowed
to migrate into the matrix. The 250 μm voxel size was larger than the
average pore size in the sample. The resulting tomograph image showed
preferential flow of water along a deformation band comprised of finegrained material.
Fig. 4. Total porosity of sandstone samples measured using neutron radiography (NRad)
compared to values obtained using a conventional method. Solid and dashed lines represent the best fit linear regression equations for ϕ b20% and N20%, respectively (De Beer
et al., 2004a).
4.2. Soils
The solid phase of soil is comprised of primary and secondary (clay)
minerals, plus organic matter (Table 2). The mineral particles are classified as sand, silt, and clay based on their size range: 0.05–2.0, 0.05–
0.002, and ≤ 0.002 mm, respectively. The specific surface area of soil
can be highly variable depending on the relative proportions of these
size fractions (Table 2). The pore spaces between soil particles are filled
with varying amounts of water and air. The total porosity of soil normally ranges between 0.17 and 0.45 (Table 2). Coarse-textured soil generally has less porosity than fine-textured soil, even though its mean
pore size is larger. Koliji et al. (2008) have used neutron tomography
to measure changes in the porosity of soil aggregates caused by external
mechanical loading. Typical values for the intrinsic permeability of soil,
corresponding to the fully-saturated condition, are given in Table 2.
Under variably-saturated conditions, permeability decreases rapidly,
and in a non-linear fashion, with decreasing soil water content. As
with rocks, neutron imaging of mobile water requires that corrections
be made to account for the presence of any hydrogen in the solid phase.
Neutron imaging has been employed to investigate both the statics
and dynamics of soil water, using NTR and NCT. The technique was
first applied to soil in the 1970's when Wilson et al. (1975) and Lewis
and Krinitzsky (1976) compared radiographic images of soil obtained
by NTR with those determined by using x-ray units. Most neutron imaging studies of soil water have employed thermal neutrons. D2O is sometimes substituted for H2O (e.g., Papafotiou et al., 2008) because it
attenuates neutrons ~ 7 × less than normal water (H2O), allowing for
the use of thicker samples.
In terms of statics, Lopes et al. (1999) applied NCT to observe the distribution of water within compacted soil. NCT has also been used to visualize and quantify static distributions of water in glass beads
(Lehmann et al., 2006). Kim et al. (2012) used NTR to study the static
distribution of thin films of water in a partially-saturated sand column.
Neutron imaging has also been employed to determine the soil
water retention curve under quasi-equilibrium conditions. Deinert
et al. (2005a) and Tumlinson et al. (2008) extracted water retention
curves from the static distributions of water within sand columns imaged using NTR and NCT, respectively. Vasin et al. (2008) obtained average drainage curves for columns of coarse and fine sand, as well as for
two heterogeneous sand columns comprised of these two sands packed
in random and periodic grid arrangements, using NCT performed under
quasi-equilibrium conditions. Fig. 5 shows changes in the 3D distribution of water within the randomly packed column during a monotonic
drainage sequence. Cheng et al. (2012) used NTR to quantify hysteresis
in the average water retention curve for a sand column under quasiequilibrium wetting and drying conditions. These authors encountered
discrepancies in NTR-determined water content measurements relative
to independent hanging water column data. The discrepancies were
similar to those reported by de Beer and Middleton (2006) and
Hassanein et al. (2006a), and were likely due to scattering associated
with large water thicknesses in the center of the saturated column.
They were effectively removed by working with relative (water) saturations rather than volumetric water contents. Recently, Kang et al.
(2014) applied NTR to determine multiple pixel-scale (or point) water
retention curves for a single sand column.
Neutron imaging has also been applied to investigate the dynamics
of water flows in soil. Clarke et al. (1987) tracked the movement of
water and development of ice lenses during soil freezing using NTR.
Brenizer and Gilpin (1987), and later Deinert et al. (2002, 2004), used
real time NTR to quantify the advance of wetting fronts into initially
dry sand columns. Tullis et al. (1994) and Tullis and Wright (2007)
used NTR to study unstable finger flows in a layered soil (fine over
coarse sand). Hincapié and Germann (2009, 2010) investigated finger
flows during gravity-driven infiltration in unsaturated sand boxes.
Gilbert and Deinert (2013) developed a method for determining radial
and vertical water content profiles within axisymmetric preferential
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
flow fields using NTR. The transient infiltration of water into packed
beds of soil aggregates (Carminati et al., 2007a,b; Carminati and
Flühler, 2009) and within the rhizosphere around plant roots (Oswald
et al., 2008; Carminati et al., 2010; Warren et al., 2013) has been the
focus of several studies using neutron radiography. In other dynamic
applications, NTR has been employed to investigate water imbibition
into granular zeolite beds (Żołądek et al., 2008) and water content dynamics during evaporative drying (Shokri et al., 2008, 2009, 2010;
Lehmann and Or, 2009; Fijał-Kirejczyk et al., 2011).
In terms of NCT, several studies have imaged and quantified the pore
scale, 3D, spatial distribution of soil water resulting from quasi steadystate flow conditions. Most of these studies have focused on artificiallypacked, heterogeneous, sand columns (Kaestner et al., 2006; Papafotiou
et al., 2008; Schaap et al., 2008). Recently, however, Badorreck et al.
(2010) have employed NTR and NCT to visualize the water flow patterns
in mine soils with natural heterogeneities.
5. Imaging studies on engineered porous media
5.1. Bricks and ceramics
Ceramic materials are man-made porous media produced from
powders by the action of heat (sintering) and subsequent cooling.
They are used in a wide range of engineering applications, including
for example semi-conductors and disk brakes. Bricks are blocks or
units of a ceramic material used in masonry constructions. Because of
their strength and durability they have been widely used as building
materials throughout history. Bricks usually contain silica sand, clay,
lime, iron oxide, and magnesium oxide. Various types of bricks (e.g.
burnt clay bricks, fire clay bricks, mud bricks, dry pressed bricks, extruded bricks, concrete bricks, and ceramic bricks) can be found in masonry
depending on the manufacturing methods used. Typical porosity values
for ceramics and bricks range from 0.19 to 0.43 (Table 2). Because of the
small pore sizes produced by the sintering process intrinsic permeabilities are usually relatively low (Table 2).
Humidity and moisture in such materials are critical to the strength
and resilience of building structures. The presence of moisture can cause
bricks and ceramics to deteriorate over time (Pleinert et al., 1998;
Nemec et al., 1999; Janz, 2002). Thus, it is not surprising that neutron
imaging has been employed since the early 1990's to evaluate the moisture status of various building materials. All of these studies have
involved dynamic imaging of water movements.
Prazak et al. (1990) first used NTR to document water uptake in
three different types of ceramic slabs: vacuum pressed ceramic,
limesand brick, and aerated concrete. They concluded that the effective
diffusivity (with dimensions of L2T−1) cannot be regarded as a material
characteristic because of its strong dependence on initial and boundary
conditions. A model, with a combination of capillary and diffusive transport mechanisms, was proposed to explain the experimental wetting
and drying profiles. Pel et al. (1993) applied dynamic NTR to determine
127
the moisture diffusivity from water content profiles measured on clay
brick and kaolin clay. Their experiments were conducted under controlled drying conditions.
Pleinert et al. (1998) employed NTR to quantify water uptake
in brick samples. The surfaces of the brick samples, which were orthogonal to the beam path, were sealed. These authors also employed
inverse numerical simulations to estimate a moisture content dependent transfer coefficient (with the same dimensions as diffusivity)
from the imaged water content profiles. Both molecular diffusion and
capillary pressure mechanisms were considered in calculating the
transfer coefficient. Islam et al. (2000) studied the water adsorption
characteristics of some Bangladeshi and Slovenian building materials
using NTR.
Buried building materials often need sealing to prevent or limit
groundwater from seeping into the base of the building. Nemec et al.
(1999) applied fast, quasi-real-time NTR to study the impregnation of
silicone-based hydrophobic agents in clay bricks. They quantified the
concentrations of two hydrophobic agents in samples and then compared the penetration of water in the treated samples. Their results indicated that moisture penetration due to capillary and diffusion
processes was evident even at long wetting times. In addition, they
noted that the relative error in the concentration profiles (about 5%)
was determined mostly by error in the calibration.
El Abd et al. (2005) utilized NTR to study the capillary motion of
water in porous construction materials including bricks (Fig. 6).
Czachor et al. (2002) employed a model, representing the porous material as a collection of capillary tubes with various radii, to describe liquid
transport. Their results for siliceous bricks suggested that the smallest
diameter capillary tubes determine the upper edge of the wetting profile based on neutron imaging data. El Abd et al. (2009) studied water
diffusivity in fired clay bricks. Acrylic paint was applied to the sides of
the samples to prevent evaporation and allow water to move in one direction. The water level in the immersing reservoir was kept constant
and covered ~ 3 mm of the immersed sample ends. Fickian diffusion
(scaling with the square root of time) was observed. In contrast, anomalous diffusion was reported for clay brick (super-diffusive) and silicate
brick (sub-diffusive) materials (El Abd and Milczarek, 2004). A power
law fit, based on the analytical model of Meyer and Warrick (1990),
seemed to be better for addressing uptake in the low water content region than the linear ratio fit. Milczarek et al. (2005) studied mass and
heat transfer in bricks using dynamic NTR and reported a selfdiffusion coefficient for water (with dimensions of L2T−1) and an imbibition rate parameter (with dimensions of LT−0.5). Exposure times for
their images ranged between 0.6 and 2.5 s. The imbibition rate parameter was found to vary linearly with the temperature. Another study
by Milczarek et al. (2008) was conducted on fired clay brick with
water in the 30–50 ° C temperature range. Their results showed a decrease in the diffusion constant to ~17% of its value for free space. The
Archie exponent, which relates to the resistance increase caused by
the pore network, was estimated to be 1.4. The Arrhenius law was
Fig. 5. Quasi-equilibrium neutron tomography images of relative (water) saturation, S, in a sand column, comprised of randomly-packed cubic inclusions of coarse and fine sand, with basal
matric potentials (from left to right) of −10, −20, −30, −40, and −50 cm during a monotonic drying sequence (Vasin et al., 2008).
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E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
applied to these data and the resulting activation energy was found to
be the same value as for bulk water. Fijał-Kirejczyk et al. (2009) studied
temperature–time–water content relations during drying of cylindrical
samples of kaolin clay using NTR.
Neutron imaging has also been used to visualize the distribution of
fluids in ceramic artifacts for archeological applications. Most recently,
Prudencio et al. (2012) applied NCT to visualize the penetration depth
and distribution of a polymer-based consolidant for the assessment of
impregnation efficiency in ancient glazed tiles. Quantification of
consolidant mass applied to ancient tiles was estimated based on the
gray level of the tomographs.
5.2. Concrete
Concrete is a composite material composed of water, aggregates including gravel, sand, and crushed rock, and cement which binds the aggregates together. The dry cement and aggregate are mixed and water is
added. As the concrete dries it hardens, increasing its compressive
strength while decreasing its porosity and permeability. Typical
porosity values for cured concrete range from 0.12 to 0.19 (Table 2).
In the absence of fractures the intrinsic permeability is typically
between 10−18 and 10−16 m2 (Table 2).
Although neutron imaging lends itself quite well to imaging static
water distributions, examples of this approach in concrete are quite
rare. Reijonen and Pihlajavaara (1972) utilized NTR to detect water produced by the carbonation process in concrete and the associated thickness of the carbonated layer. Howdyshell (1977) determined the water
content of fresh concrete using NTR. Other static hydraulic properties of
concrete estimated using neutron imaging include total porosity by NTR
(Zeilinger and Huebner, 1976; de Beer et al., 2004b; de Beer et al., 2005;
de Beer and Middleton, 2006) and NCT (Brew et al., 2009; and McGlinn
et al., 2010), and pore-size distribution (Brew et al., 2009; McGlinn et al.,
2010) by NCT. Pugliesi and Andrade (1997) used NTR to visualize cracks
in concrete samples that had been subjected to a compressive strength
test. In order to enhance crack visualization, the samples were wetted
with an aqueous solution of gadolinium nitrate, and then dried.
Dynamic applications found in the literature deal primarily with
visualizing and quantifying changes in water content over time due to
either the drying of wet concrete or the movement of external water
into dry concrete. de Beer et al. (2004b) employed NTR to observe the
redistribution of water during the natural drying process immediately
following curing. Saturated lightweight aggregates can serve as reservoirs during the drying process, transferring water from the aggregate
Fig. 6. Raw neutron transmission images showing the horizontal movement of water
(dark gray) into initially dry columns of: (a) clay and (b) siliceous bricks (light gray) at different times (©2005 IEEE. Reprinted, with permission, from El Abd et al., 2005).
to the surrounding matrix. This transfer of water was investigated by
Maruyama et al. (2009), using NTR, and Trtik et al. (2011), using NCT,
with similar results. Both studies showed water migration of at least
3 mm from the aggregate into the surrounding matrix.
Zeilinger and Huebner (1976) visualized moisture migration
when heat was applied to one end of a moist sample. The authors calculated the vapor diffusion coefficient and the mass transfer coefficient
based on the acquired moisture profiles. More recently, Milczarek
et al. (2005) investigated vapor transport due to boiling of water within
a concrete sample.
When wetting by capillary suction is utilized, the sorptivity of
the sample is typically calculated. Hanziç and Illic (2003) determined that the relationship between the height of capillary rise
(or the volume of liquid absorbed per unit area) and the square
root of time only holds true for times b 60 h. de Beer et al. (2004b)
and de Beer et al. (2005) compared traditionally measured porosity
and sorptivity values with those obtained by NTR and found good
agreement between the methods. Brew et al. (2009) provide another
example where sorptivity values obtained by NTR agreed well with
those obtained by the traditional gravimetric method. In this study
the authors attempted to correct for neutron scattering by applying
a Monte Carlo model based on the Point Scattered Function approach
developed by Hassanein et al. (2006a). McGlinn et al. (2010) investigated the rate of water penetration into dry concrete samples, with
capillarity as the driving force, using neutron tomography.
In other studies, changes in moisture content and distribution due to
water entering fractures have been the primary concern. Kanematsu
et al. (2009), Wittmann et al. (2010) and Zhang et al. (2010a,b, 2011)
investigated water uptake into cracked concrete and its effects on the
deterioration of steel reinforcements (Fig. 7). In the case of reinforced
or strain-hardened concrete, lower water to cement ratios may lead to
early cracking of the concrete.
Other dynamic hydraulic properties of concrete estimated using
neutron imaging include the permeability of a sample within a pressure
cell (Dawei et al., 1986), unsaturated diffusivity (Prazak et al., 1990),
and the uptake of water when a hydrophobic agent has been applied
to the concrete (Zhang et al., 2010a,b, 2011).
5.3. Fuel cells
A fuel cell is an electrochemical energy conversion system that generates electricity from chemical reactions. The style of fuel cell most typically imaged with neutrons is the low temperature polymer electrolyte
fuel cell which operates below 90 °C allowing liquid water to exist. This
device can contain several layers of porous material. The layer of interest for this review is the gas diffusion layer (GDL) which provides liquid
water removal from the catalyst layers to the gas channels while
allowing for even gas distribution along the catalyst. The GDL typically
ranges in thickness from 100 to 400 μm and is constructed of carbon fibers with diameters of ~10 μm. Total porosity and intrinsic permeability
values for GDL's are generally quite high (Table 2). There are two basic
forms of the GDL: paper and cloth. The paper GDL is a stiff material
formed by the random orientation of carbon fibers that are bonded together through a graphitization process. The cloth GDL is a more flexible
material constructed with woven bundles of fibers. GDL's are often coated with a layer of polytetrafluoroethylene (PTFE) to make the pores hydrophobic. However, variability in the coating process can result in a
heterogeneous distribution of hydrophilic and hydrophobic regions
within the layer.
Fuel cells continuously produce water during operation that must be
removed from the cell. Due to the dynamic nature of this process no
studies have been found that deal with the statics of water in the GDL.
Instead high spatial resolution NTR is employed to elucidate changes
in the liquid water saturation of the GDL based on changes in operating
conditions, such as temperature, current density, pressure, and humidity. Testing is typically performed at constant current operation which
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
allows for constant water production. High resolution NTR permits the
use of standard fuel cell materials such as aluminum and graphite.
This allows for more realistic thermal boundary conditions on the cell
when compared to x-ray radiography which achieves higher resolution
than NTR but requires materials such as polymers that can alter temperature gradients in the cell.
One of the earliest examples of neutron imaging of fuel cell diffusion
media was the study by Satija et al (2004). However, due to limited detector resolution the in-plane direction of the fuel cell was imaged
which agglomerates all of the liquid water in the cell together. This
makes it difficult, if not impossible, to uncouple the liquid water content
of each of the individual layers. In-plane imaging has been the focus of
several papers such as Turhan et al. (2008), Cho et al. (2009), Owejan
et al. (2009), and Spernjak et al. (2009) to list a few. It wasn't until the
advent of high-resolution detectors in the range of 25 μm that
through-plane imaging was made possible. This new approach allows
for the detection of saturation profiles through each individual layer.
Hussey et al. (2007) focused on the initial testing of a high spatial resolution detector capable of imaging the through-plane direction of the
fuel cell. Other groups have also presented new detectors and techniques with initial images of fuel cell water contents (Boillat et al.,
2008a,b; Lehmann et al., 2009; Boillat et al., 2011; Murakawa et al.,
2011; Yasuda et al., 2011; Mishler et al., 2012). While the majority of
imaging facilities used for fuel cell research are thermal beams which
allow for greater material penetration, some researchers such as
Boillat et al. (2008a) have used cold imaging lines to improve contrast
of small quantities of water.
One of the first quantitative analyses of through-plane water content
dynamics in fuel cells was conducted by Hickner et al. (2008). These authors developed a specialized fuel cell adapted for high resolution NTR.
The aim of this study was to investigate the effects of inlet gas humidity,
cell temperature, gas flow rate, and current density on liquid water profiles through the cross section of the cell. Condensation was found to
occur in the GDL as the microporous layer restricts liquid transort.
This was inferred by the maximum in water thickness found at the center of the GDL as shown in Fig. 8. The conclusion by Hickner et al. (2008)
of vapor transport to the GDL, where water then condenses, was later
modeled by Weber and Hickner (2008). The simulations revealed a
strong heat-pipe effect (see Section 5.4) within the fuel cell. This effect
Fig. 7. Neutron radiograph showing preferential upward movement of water (dark gray)
into fractured reinforced concrete (light gray) after one hour of wetting (adapted from
Zhang et al., 2011).
129
occurs when water transport is augmented by a temperature gradient
in the direction of flow.
Kim and Mench (2009) developed a fuel cell based on the hardware
used by Hickner et al. (2008) and Weber and Hickner (2008) that
allowed for more precise control of the thermal boundary conditions.
The anode and cathode temperatures were controlled independently
with heating/cooling circulators. Temperature gradients were then applied to the cell while neutron imaging took place. Ex situ tests were
conducted where the side channels were filled with water and images
were taken to determine leakage rates. Flow only occurred when the
hot side was on the water side and the cold side was on the dry side.
Hatzell et al. (2011) utilized the same cell configuration as Kim and
Mench (2009) to test the influence of the temperature gradient on
phase-change induced flow. It was found that water transport increased
with increasing temperature gradient. Ex situ tests were used to provide
further validation that the microporous layer inhibits liquid water
transport.
Turhan et al. (2010) probed the effects of gas channel surface energy
on GDL water content. Tests were conducted with hydrophobic and hydrophilic treated flow fields. The investigators found that the hydrophilic channels helped to pull water out from the GDL above the channel
ribs, but this made drying during purge more difficult. Tabuchi et al.
(2010) tested a small 1 cm2 cell with multiple flow field arrangements.
Straight channels with different rib channel widths were tested to determine how the channel/rib ratio affected the cross-sectional water
distribution in the GDL. Larger rib sizes were found to reduce cell performance due to the collection of water in the GDL above these areas. This
increase in liquid water content reduced gas phase transport limiting
reaction rates. Cho and Mench (2010) employed high-resolution NTR
to determine the drying effectiveness of different purge cycles as a function of the channel/rib ratio. Their results showed that a composite
purge cycle, comprised of an initially high flow rate to remove large
water droplets followed by a low flow to dry the GDL by evaporation,
provided the optimal condition for low energy purge and start-up
reliability.
Manahan et al. (2011) proposed changes to the GDL to increase fuel
cell performance. These researchers used an ytterbium fiber laser to
perforate the GDL with larger (300 μm) diameter pores to facilitate
water movement. NTR indicated the perforations enhanced cell performance under low humidity inlet gas and low current density operation,
b1.4 A/cm2. However, above this current density and at high humidity
conditions, the perforations had a negative effect on cell performance.
The larger pore spaces collected and retained water most likely due to
the lack of hydrophobic coating in the heat affected zone around the
perforations. Water exchange between freshly generated water and accumulated water within the cell can be measured by hydrogen–deuterium contrast neutron radiography. Manke et al. (2008) collected
initial radiographs of a fuel cell running on hydrogen gas after which
the gas stream was switched to deuterium. Once the switch occurred,
any freshly generated water would be heavy water which is nearly
transparent to neutrons compared to light water. Due to the contrast
difference between light and heavy water, the attenuation from water
in the cell would decrease as light water was replaced with heavy
water. It was found that at low current densities a simple one-phase
convective model was sufficient to predict diffusion. At higher current
densities this model did not agree with experimental data showing
that a faster water removal process, similar to Haines jumps, was
present.
Boillat et al (2008b) used hydrogen–deuterium labeling to measure
exchange rates between hydrogen gas and protons in the membrane.
It was found that the exchange rate was higher than literature values
likely due to a higher exchange current density than is measured by traditional means. Cho and Mench (2012) investigated the role of microporous layers (MPL) with hydrogen–deuterium contrast. It was found that
light water replacement occurred primarily on the cathode side where
water is generated with no MPL but changes occurred on both sides
130
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
Fig. 8. Neutron radiographic cross-section through an operating fuel cell. Low water contents are denoted by the blue/purple colors, high water contents are denoted by the pink/
red colors. The pixel column numbers are labeled for the approximate locations of the center of the membrane (212), the macroporous layer/microporous layer interfaces (205 and
219), and the boundaries of the GDL (185 and 235). The anode gas flow channels are to the
left of pixel column 185, and the cathode gas flow channels are to the right of pixel column
235 (Hickner et al., 2008; Reproduced by permission of ECS - The Electrochemical Society).
neutron imaging of a working heat pipe had been done (Matsumoto
et al., 1986). Others attempted to quantify vapor distribution by looking
at vaporization within the wick structure (Moss, 1967; Balasko et al.,
1986), as well as water thickness and vaporization within different
wick structures at various inclinations (Moss and Kelly, 1970).
Cimbala et al. (2004) used NTR to visualize partial wick dry-out and distribution problems in the cooling water system, which can limit the performance of a heat pipe system (Fig. 9).
A study done by Yoon (2008) looked at the dynamics of vapor and
liquid flow in oscillating heat pipes by comparing volume fraction data
with temperature data at various points in the heat pipe. Wilson et al.
(2008) compared flow patterns of aqueous solutions containing differing amounts of diamond nanoparticles and observed the circulation of
vapor bubbles and fluid plugs. Shifts in the filling ratio of an oscillating
heat pipe system were found to induce changes in flow motion and
heat transfer (Borgmeyer et al., 2010). Sugimoto et al. (2009) imaged
a self-vibrating heat pipe consisting of a meandering capillary channel
with butane as the working fluid. The behavior of the butane was observed by video imaging at 200 frames per second. Vibrations were generated (due to the pressure differential) as heat was transported from
high- to low-temperature regions.
Liquid plug formation, which can block vapor passage and decrease
performance, has been investigated by Sugimoto et al. (2011) and
Wilson et al. (2011). Sugimoto et al. (2011) eliminated a liquid plug
formed in a bend in the heat pipe tubing by inserting a thin metal
plate. Thompson et al. (2011) studied the effects of various Tesla-type
valves on circulation and flow behavior in an oscillating heat pipe, concluding that the addition of the Tesla-type valves promoted circulatory
flow and lowered thermal resistance.
5.5. Porous glass
with MPL present. This indicates that the MPL provides a barrier to
water removal on the cathode and thus pushes water towards the
anode.
Three-dimensional neutron tomography has been used to decouple
channel and diffusion media water content of ex situ (no reaction occurring) fuel cells by Sakata et al. (2009), Tang et al. (2010), Takenaka et al.
(2011), Markötter et al. (2012), and Santamaria et al. (2012). Due to the
length of time required to acquire all of the images from 0 to 180° of
rotation to reconstruct the fuel cell, the fuel cells are usually shutdown
to keep water in place. Image resolution is typically in the order of 75
to 200 μm for this technique to allow for high temporal resolution. Because spatial resolution is on the order of the diffusion media thickness,
channel water content is of primary interest in these studies.
5.4. Heat pipes
Heat pipes are devices for transferring heat from one location to another based on the phase change of a working fluid (often water). The
working fluid evaporates to remove heat from the hot end; the vapor
travels the length of the tube where the heat is dissipated at the cold
end through condensation. To produce a closed cycle where the working fluid is reused, a porous wick is added to the inside of the pipe to
move the condensed working fluid from the cold end back to the hot
end through capillary action. Wicks for heat pipes are made out of
metal, carbon fibers, polymers, or ceramics with a sintered powder,
mesh, or grooved structure along the interior walls. They can have a
large porosity range, from as low as 0.15 to as high as 0.75, with permeability values on the order of 10−9 to 10−13 m2 (Table 2).
Heat pipes are inherently dynamic in their operation and, perhaps
not surprisingly, the literature contains no examples of the use of neutron imaging to visualize the static distribution of water within a wick
structure. Instead the main research focus has been on dynamic NTR
of fluid flow patterns and the distribution of vapor in wicks. Some initial
studies were qualitative in their analysis, looking at the spatial distribution of the working fluid (Tamaki et al., 1986), or only mentioning that
Porous glass contains ~96% silica (as compared to common quartz
glass, fused quartz, ~63% silica) and features an interconnected porous
microstructure (Table 2). It is one of the most common nanoengineered materials along with carbon, silicates (zeolites), and polymers. Porous glass is generally made through a process of phase separation and liquid extraction. It can be produced in high quality with mean
pore sizes of ~0.4 nm up to ~1 μm, and with a very narrow distributional
range (Table 2). Due to its unique properties (i.e., high chemical, thermal
and mechanical resistance) and functionalization of the inner surface,
porous glass is used for a wide range of applications in science and engineering (Elmer, 1992; Gelb and Gubbins, 1998; Yao et al., 2003).
To date only one study has appeared in which neutrons have been
utilized to image water within porous glass. Gruener et al. (2012) applied neutron radiography to investigate the anomalous behavior of
the wetting front width during spontaneous imbibition of water in a
nanoporous glass (Vycor) with elongated pores. The quantified data
from the neutron imaging were compared against results from a theoretical pore-network model and numerical simulations. The wetting
front followed the well-known square root of time scaling. However,
the interface width was strongly dependent upon the pore aspect
ratio (Fig. 10). Large values of this ratio (i.e., more elongated pores) inhibit the formation of a connected vapor–liquid interface and lead to a
rapid broadening of the imbibition front. These authors also noted
that neutron imaging permits observation of the advancing front deep
within the matrix, which is usually difficult to do by other means
(e.g. NMR).
6. Discussion and future directions
It has been shown that neutron imaging (by transmission radiography and/or computed tomography) can be a powerful non-destructive
tool for visualizing hydrogen-rich fluids within diverse porous media
(both natural and engineered), under static and dynamic conditions.
The value of neutron imaging in determining physical and hydraulic
E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
131
Fig. 9. Neutron radiograph of an operating heat pipe. The indentations (indicated by
arrows) on either side of the evaporator show that liquid water (dark gray) has been
replaced by vapor (light gray) signifying partial wick dry-out (Cimbala et al., 2004).
properties of variably-saturated materials has been demonstrated with
numerous examples drawn from the literature. While many of the early
studies were essentially proof-of-principle in nature, yielding mainly
descriptive datasets, neutron imaging is increasingly being used to extract detailed quantitative information at the pore/pixel scale of resolution. Properties that have been measured, or should be quantifiable, by
means of neutron imaging include total porosity, pore-size distribution,
fluid content and distribution, capillary pressure–saturation parameters, liquid flow and transport parameters, such as sorptivity and hydraulic diffusivity, the gas diffusion coefficient, and soil compaction
characteristics, amongst others. However, there is an urgent need for
the establishment of appropriate standards for digital neutron radiography and tomography to instill confidence in such measurements
(Kaestner et al., 2013).
In terms of new research, there is always the desire for increased
spatial and temporal resolution with respect scintillator/camera capability. The current state-of-the-art for temporal resolution is in the
sub-μs range (Siegmund et al., 2009; Tremsin et al., 2013), which is sufficient for most applications involving flow of hydrogen-rich fluids in
porous media. In terms of spatial resolution, the current state-of-theart is ~15 μm, and the push is on to get down to 1 μm (Tremsin et al.,
2008; Siegmund et al., 2009). Unfortunately, for many natural and
engineered materials even 1 μm spatial resolution will be insufficient
to visualize fluid distributions within individual pores. This is certainly
the case for nanoporous materials which represent a rapidly-growing
area of interest. As a result, researchers are forced to work with images
that represent only average saturations at the micro-scale (e.g., Gruener
et al., 2012). In this context, the spatial resolution target for the neutron
imaging community should be the sub-μm range, although it is uncertain if such a goal can be achieved in the foreseeable future.
In conjunction with advances in imaging resolution, the problem of
scattering from samples containing large amounts of water (either
because they are fully saturated or very thick) will need to be resolved.
Scattering can result in the significant under estimation of water content (or porosity) as illustrated in Fig. 4. Since most imaging studies
involving variably-saturated porous media require data collection over
a broad range of fluid contents, and from sample volumes sufficient to
account for heterogeneity, focusing only on small sample sizes and/or
low water contents is too restrictive. Modeling the point scatter function
and/or the development of beamline specific calibration equations are
currently the best options available. However, there is a need to bolster
these two approaches with new imaging configurations that permit experimental determination of the scattering components.
Combining images obtained using both neutron and x-ray imaging
could be very beneficial for future research (Banhart et al., 2010).
These techniques currently overlap at 0.1 to 1 mm length scales in
terms of sample size and image resolution (Kaestner et al., 2008). Neutron imaging provides detailed information on the distribution of water
Fig. 10. Progression and broadening of the imbibition front of water in nanoporous Vycor
glass (with a pore aspect ratio between 5 and 7) as quantified by neutron imaging. Graph
shows the laterally averaged degree of pore filling as a function of height and time. The
square root of time behavior of the wetting front advance is shown as a solid line (Gruener
et al., 2012).
within a porous medium, but cannot see the air or solid phases. In contrast, x-ray imaging provides detailed information on the geometry of
pore-solid interfaces within the matrix. By stitching together images obtained from both techniques it should be possible to visualize the distribution of fluids with respect to individual pore bodies and necks. Such
images would be very valuable in studies of multiphase flow and
transport.
Many natural porous media contain hydrogen within their mineral
matrix, along with naturally-occurring organic matter. To date, however, most neutron imaging studies of water in rocks and soils have focused on samples that do not contain much native hydrogen. As we
move forward with imaging studies on a wide range of natural materials, however, there will be a need to conduct additional detailed calibrations to account for the presence of this matrix hydrogen. In fact,
the method appears to be ideally-suited for visualizing the fine scale
spatial distribution of organic matter in soil aggregates, an important
topic for soil science, but one that does not yet appear to have been investigated using neutron imaging. In geology and petroleum engineering, hydrogen-rich fluids, such as oil, natural gas, and methane in rock
reservoirs are of major importance as they relate to our energy economy. While some research has already been conducted on imaging
such fluids (e.g., Middleton and Pàzsit, 1998; Solymar et al. (2003b),
this application also appears to be ripe for further detailed investigations, especially given the rapid rise to prominence of non-traditional
extraction methods such as hydraulic fracturing.
Finally, we would be remiss if we did not mention the exciting opportunities offered by multispectral neutron imaging (also known as
time-of-flight or energy selective neutron imaging). In this type of imaging, images are acquired at specific energy bands. Then, by “tuning” the
images across energy bands, it becomes possible to discriminate, for example, between the liquid and solid phases of water (Lehmann et al.,
2011a). This is an important capability since information on the spatial
distributions of unfrozen liquid water and ice within a porous medium
at sub-zero temperatures is urgently needed in applications as diverse
as permafrost thawing and the performance of fuel cells. While it is possible to perform energy selective neutron imaging at continuous polychromatic neutron sources using wavelength selectors (Treimer et al.,
2006), pulsed spallation sources, such as the existing SINQ neutron imaging facility at PSI, Switzerland, and the SNS-VENUS, ISIS-TS2, ESSODIN, and J-PARC-ERNIS instruments currently under development in
the US, UK, Sweden, and Japan, respectively, offer the best prospects
for progress in this area.
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E. Perfect et al. / Earth-Science Reviews 129 (2014) 120–135
Considering current beam line capabilities and the prospects for future advances in resolution and energy selective imaging we are very
excited about the future for neutron imaging and its applications to natural and engineered porous materials. We believe this non-destructive
technique will play an increasingly important role in advancing porescale understanding of the phase structure and flow of hydrogen-rich
fluids in variably-saturated porous media.
Acknowledgements
This review is based in part on assignments prepared by graduate
students participating in a seminar course (GEOL 685) on imaging
water in porous media taught by the first author at the University of
Tennessee - Knoxville (UTK) in the spring semester of 2012. Funding
was provided by the Laboratory Directed Research and Development
Program of Oak Ridge National Laboratory (ORNL) and the Joint Directed Research and Development Program of the UT-ORNL Science Alliance
at UTK.
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